As shown in FIG. 9, a magnetic circuit incorporated in a conventional electromagnetic-force-balancing-type (or electromagnetic-force-compensation-type) electronic balance comprises first and second permanent magnets 1, 2 disposed such that their magnetic poles of the same polarity are opposed to one another, a pole piece 3 sandwiched between the first and second permanent magnets 1, 2, a tubular-shaped yoke 4 having an opening only on one side thereof and an inner surface in contact with the first permanent magnet 1, and a pair of covers 5a, 5b in contact with or adjacent to the second permanent magnet 2 (see, for example, the following Patent Publication 1). The electromagnetic-force-balancing-type electronic balance has a movable assembly 10 which comprises a movable lever 11, a force-coil mounting plate 12 fixed to one end of the movable-section lever 11, and a force coil 13 fixedly mounted to the force-coil mounting plate 12. The electronic balance also has a weighting pan (not shown) fixed to the other end of the movable-section lever 11. The electronic balance is designed to control a current to be applied to the force coil 13 so as to allow the force coil 13 to be kept at a position approximately concentric to the pole piece 3, and subject a current value detected from the force coil 13 to various corrections so as to calculate a weight of an object placed on the weighting pan.
In a design stage of the magnetic circuit, with a view to reliably fastening between the cover pair 5a, 5b and the yoke 4 through screwing or the like, regardless of machining tolerances and assembling errors in the components, an air gap A has been set between the cover pair 5a, 5b and the second permanent magnet 2, as a margin of error (see, for example, the paragraph [0024] of the Patent Publication 1).
[Parent Publication 1] Japanese Patent No. 3691607
The air gap A is likely to be entirely or partly left only on the side of the second permanent magnet 2 after assembling of the magnetic circuit. The remaining air gap creates a magnetic resistance in the magnetic circuit to cause a problem, such as hysteresis. This problem will be specifically described with reference to FIG. 10, wherein FIG. 10(a) is a sectional side view showing the magnetic circuit incorporated in the conventional electromagnetic-force-balancing-type electronic balance, and FIG. 10(b) is an equivalent magnetic circuit thereof. As shown in FIG. 10(a), there are two magnetic sub-circuits: a first magnetic sub-circuit 21 extending from one of the magnetic poles of the first permanent magnet 1 to the other magnetic pole of the first permanent magnet 1 through the yoke 4 and a space between the yoke 4 and the pole piece 3; and a second magnetic sub-circuit 22 extending from one of the magnetic poles of the second permanent magnet 1 to the other magnetic pole of the second permanent magnet 2 through the cover pair 5a, 5b, the yoke 4 and the space between the yoke 4 and the pole piece 3.
Respective equivalent magnetic sub-circuits 23, 24 of the first and second sub-circuits 21, 22 are shown in FIG. 10(b). If a certain air gap exists between the cover pair 5a, 5b and the second permanent magnet 2 in the second magnetic sub-circuit 22, a magnetic resistance RA proportional to the air gap will be created in the second permanent magnet 2 to cause an imbalance between the first and second magnetic sub-circuits 21, 22. As to this problem, the inventors found that the influence of hysteresis on change in current flowing through the force coil 13 becomes more prominent as a current to be applied to the force coil is increased or as a required torque for the movable assembly 10 is increased, as described in more detail below.
In response to a magnetic field externally applied to a ferromagnetic material, such as iron, magnetic domain walls are moved to produce magnetization in a direction of the applied magnetic field, and the number of magnetic domains oriented in the magnetic field direction is increased to generate magnetization. Then, when the magnetic field intensity is further increased, the entire crystal structure of the material has only magnetic domains oriented in the magnetic field direction, and the magnetization reaches saturation. In this process, when the ferromagnetic material is a high-purity metal, the magnetic domain wall movement can be induced easily, or the magnetization reaches saturation only by a low intensity of magnet field. In contrast, if the ferromagnetic material contains impurities, the magnetic domain wall movement is hindered, and a higher intensity of magnetic field is required to allow the magnetization to reach saturation. Moreover, even after the external magnetic field is eliminated, the magnetization intensity will not return to zero to cause a remanent magnetization. As with the ferromagnetic material containing impurities, a remanent magnetization is caused by the air gap A existing in the magnetic circuit. The yoke 4 or the cover pair 5a, 5b is magnetized by the second permanent magnet 2. Further, during the course of magnetization based on a magnetic field generated by the force coil 13 applied with a current, when the applied current is relatively low, a resulting magnetization intensity is limited to a small ratio relative to the magnetization intensity based on the permanent magnet, or to a small value relative to the saturation magnetization, and therefore the intensity of a resulting remanent magnetization will have a negligible small impact. In contrast, if a current is applied to the force coil 13 at a value allowing a magnetization intensity to reach saturation, a resulting remanent magnetization or hysteresis will have a non-negligible impact.